1. Field of the Invention
This invention relates to illuminating and reading optical codes using optical code reading devices. In particular, this invention relates to a system and method for illuminating and reading optical codes imprinted or displayed on reflective surfaces.
2. Description of the Prior Art
As industry has continued to refine and improve production techniques and procedures, corresponding requirements have been levied for placing identifying data related markings upon components of manufactured assemblies. These markings enable tracking of, for example, the historical stages of a product's manufacture. Further, these markings enable components of complex machinery, such as automobiles and the like, to be identified, for example, in the course of an investigation by quality control personnel or governmental authorities.
A variety of product marking approaches has been utilized in industry. For example, paper tags or labels carrying UPC codes are typically applied to components in the course of a product's assembly. This method provides identifying codes that are highly readable by prior art optical code readers. However, for many applications, such tags or labels may be lost, damaged or altered—rendering the coding useless. This shortcoming is well addressed by Direct Part Marking (DPM) and consequently DPM has gained widespread acceptance in industrial applications by providing durable markings, or code symbols, capable of being placed directly on a wide range of surfaces.
DPM is the technique of directly imprinting, etching or dot-peening (indentation marking) product and component surfaces with any of a number of symbologies—particularly high-density 2-D code symbologies, such as Data Matrix and QR Code, but even commonly used UPC codes are produced in this manner. However, as a result of being formed directly onto a wide variety of materials and textures, proper reading (and subsequently decoding) of DPM codes poses a challenge. Readability of DPM codes is typically a function of contrast between the code symbol and the background surface. The DPM method does not typically provide for the selection of background color or foreground color or reflectivity of the symbol markings, especially in the case of etching and dot-peening. Therefore, DPM markings often have low and inconsistent contrast, typically resulting in limited code readability.
Additionally, applications exist for displaying optical codes on screens, such as CRT or LCD displays, such as a cell phone display. For example, a consumer transaction may be performed using a cell phone where a consumer uses the cell phone to purchase a ticket, such as an event ticket or a lottery ticket, including making payment via the cell phone and receiving the purchased ticket as an electronic ticket through the cell phone in the form of a message bearing a barcode which is displayable by the cell phone. Upon redemption of the ticket, the electronic ticket's barcode displayed on the cell phone is scanned by the merchant redeeming the ticket.
Generally, prior art optical code scanners are designed for scanning paper-based optical codes. Optical codes, such as UPC symbols, printed on labels provide high contrast through selection of background and foreground colors—usually black geometrical shapes or foreground on a white background. Furthermore, paper's superb omni-directional scattering property covers up much of the traces of the structure in the light source.
However, many objects marked with a DPM code and display screens (e.g., mobile phone, CRT, LCD, plasma, etc., displays) displaying barcodes may have surfaces of entirely different characteristics. In some cases, a smooth surface may form a reflected image of the light source and cause the optical code symbol to appear washed-out or even indiscernible from its surroundings; in such circumstances, an extended light source, providing a large and uniform emission surface, would be desirable. Further, the precise angle of the originating light source, with respect to the DPM markings, needs to be controlled in order for variations in surface texture created by the marking method to reflect the light with different intensities. Surface background conditions that are reflective, or yield very little visible symbol contrast to the naked eye under general room lighting, can become highly visible when illuminated by a light source at a given angle from the surface. In such circumstances, directional (or direct-point) illumination is desirable.
One commercially available prior art DPM scanner (see Prior Art FIG. 1a) provides an extended light source and covers multiple angles. This prior art scanner is equipped with a lens 100 positioned in an optical path between an optical code (not shown) and an image sensor 105. The optical code is illuminated by a set of LEDs 101 situated behind a diffusing plate 102. The diffusing plate 102 transmits the light produced by the LEDs 101 as diffuse illumination instead of direct illumination. Additionally, a diffusing tube 103 prevents ambient light from illuminating the optical code, thus the only illumination applied to the optical code originates from the diffusing plate 102. Baffles 104, configured to prevent stray light from escaping the diffusing plate 102 directly into an aperture 120, are positioned around the aperture 120 leading to the lens 100.
However, this system has two limitations of particular interest with respect to the present invention. First, this system is relatively large, making the scanner cumbersome and generally unsuitable for integration with handheld computing devices. Second, the translucent diffusing plate used by this system has to be relatively thick to be effective for providing a large and uniform emission surface. The overly thick diffusing plate, however, is inefficient for transmitting light, and consequently requires more powerful illumination than would be necessary with by a light source.
In other instances, DPM images obtained using direct point light sources, or alternatively narrow beam light sources, produce better contrast, especially for DPM marks made using dot-peening methods—the method of repeatedly impacting the part surface with a sharp tool, thus, forming indentations on the surface. The indentations tend to reflect light at angles substantially similar to the incident angles of the illumination source. Another commercially available DPM scanner (shown in Prior Art FIG. 1b) combines the extended light source method described above and illustrated in FIG. 1a with a direct point light source method. The scanner depicted in FIG. 1b is equipped with a first and second set of LEDs 107. The first set of LEDs 107 is positioned close to the lens 106 for providing direct illumination of the optical code. The second set of LEDs 107 lying on the cover/diffusing plate 108 provide diffuse illumination to the optical code. This system allows multiple illumination sources to be utilized to provide the image sensor 109 with a decodable image of the optical code. However, this scanner is even larger than the extended light source DPM scanner described previously and illustrated in FIG. 1a. 
Other commercially available systems include several banks of LEDs that are switched on at different times to illuminate the DPM markings from various directions using the same color. The image having the highest contrast is then selected for decoding. The contrast in the images obtained varies with each illumination direction; successful decoding depends on properly illuminating the DPM code or marking from at least one of the illumination directions.
Another system is available in which an extended light source is provided at a substantial angle to a reflective diffuser which reflects the light emitted by the extended light source for redirecting the light to an optical code being imaged. The angle of the extended light source and the reflective diffuser relative to one another and to a front face of the device creates the need for the device to have a substantial depth and to be relatively cumbersome.